ELECTRICAL TREE GROWTH
IN DC INSULATION
BACKGROUND
Electrical trees are defects which develop in high
voltage polymeric insulation, and are precursors to
irreversible failure. They resemble botanical trees, but
are bifurcated tubule structures. The phenomenon is
understood to a degree in AC insulation systems,
allowing conservative design criteria for cables and
HV insulation systems to be established. However,
knowledge of their growth in DC systems is very
limited. Boundaries and interfaces between insulating
materials are also known to influence defect growth
and eventual failure, but this has not been explored in
DC systems.
AIMS AND OBJECTIVES
 To understand how space




charge accumulates at
dielectric interfaces
To develop methods
enabling reproducible
growth of electrical trees
To determine whether
electrical trees will develop
under ‘pure’ DC stress
To determine the importance
of AC ripple, or power
quality, on DC tree growth
To provide improved
quantitative methods for
physical tree
characterisation
Needle-plane geometries
were used to grow trees
in epoxy resin
RESEARCHERS
Prof Simon Rowland,
Ibrahim Iddrissu,
Dr Antonios Tzimas,
The University of Manchester
The University of Manchester
The University of Manchester
now with Alstom
Dr Roger Schurch,
The University of Manchester
and Universidad Técnica Federico Santa María, Chile
s.rowland@manchester.ac.uk
Development of a tree growing through epoxy insulation
exposed to AC superimposed on positive DC stress. The
sample failed after 47 minutes.
ACHIEVEMENTS
It has been shown that DC stress with superimposed
power-frequency AC leads to faster growth than with 50
Hz AC alone. Moreover it has been demonstrated that
positive DC bias leads to earlier breakdown than
negative bias. It is suggested that the negative bias is
less onerous as a result of field moderation by homospace charge at the negative needle electrode.
Stage1
Stage 2: Tree Propagation
Stage 3: Runaway
103
102
10
1
Region 3
Region 2
Region1
Tree Inception
Charge (pC)
It is well established that polarity reversal and impulse
voltages lead to electrical tree initiation and growth in
polymeric insulation under DC stresses. By using
extremely clean power supplies we have shown that
trees can also initiate and grow under purely DC
stresses. In practical terms, in networks, tree growth is
likely to be initiated by anomalous events associated
with power quality, but we have shown that growth is
also associated with long term DC exposure.
Region 4
BD@120min
100
10-1
1min
0
89min
47min
15min
20
40
60
80
199min
100
120
140
Time (min)
Measurement of physical growth and partial discharge
activity allow four stages of growth to be identified.
Use of advanced imaging techniques, including XCT at
the UK Diamond synchrotron and the Paul Scherrer
Institut in Switzerland has enabled unique 3D images of
trees to be developed.
IMPACT
This core work, enabled by Top and Tail, has led to two
new projects being funded by EPSRC to take the
understanding of treeing further in DC systems, and
structured AC insulation: ‘Towards Enhanced HVDC
Cable Systems’ EP/L021560, and ‘Composite Dielectric
Structures with Enhanced Lifetimes’ EP/M016234.
An example publication is: ‘Comparison and
Combination of Imaging Techniques for Three
Dimensional Analysis of Electrical Trees’, R. Schurch, S.
M. Rowland, et al, IEEE Transactions on Dielectrics and
Electrical Insulation, Vol. 22, pp. 709-719 (2015)
3D renderings of an electrical tree. Images on the right are
magnified versions of branches from the models on the left.